Monitoring the earth’s volcanoes from space poses a variety of unique problems and opportunities related to spatial and temporal scales, as well as spectral range (e.g., the necessity for thermal IR observations). Some of these are well posed with respect to the ASTER instrument and mission, and some challenge ASTER’s limitations.

Introduction

Volcanoes represent one of the most active features of landscape generation. The frequency of discernable volcanic feature generation (as opposed to indiscernible fault or landslide/glacial creep) is exceeded only by aeolian-generated landforms and beach landforms (at times constantly changing at an observable spatial scale). Volcanic eruptions reported during human history range in frequency from daily (e.g., Mt. Etna, Italy; Mt. Sakurajima, Japan) to once every several centuries (e.g., Mt. Pinatubo, Philippines; Mt. St. Helens, USA). Currently there are of order 1000 volcanoes worldwide that are considered active, in the sense that they could enter a restless state or erupt more-or-less at any time (Simkin and Siebert, 1994).

The earth’s volcanic activity manifests itself at fairly well-defined spatial scales. Lava flows typically exhibit characteristic length scales of 1-10km, with exceptional flows reaching length scales of the order of 100km (e.g., ancient high temperature komatiite flows). Central vent volcanic features on the earth have characteristic base diameters of the order of 10-100km, with basaltic shields tending toward the high end and more compact stratovolcanoes tending toward the lower end of the range. Thus, ASTER’s 60x60km footprint and 15-30-90m/pixel (VNIR-SWIR-TIR) resolution scale allow most central vent volcanoes to be captured in just a few frames centered on the feature, and in many cases with just one image. At 15m/pixel (VNIR) and 30m/pixel (SWIR), ASTER images reveal the spatial distribution of lava flow and summit crater hotspots (Figures 1 and 2). Landsat ETM+ can provide comparable imaging and hotspot assessments; however, its dynamic range in the SWIR region is less than ASTER.

Figure 1. A false color composite of the three ASTER VNIR channels showing the 29 July 2001 summit eruption of Mt. Etna. Spatial resolution is 15 m/pixel. Areas appearing red are vegetated. Strong water vapor, ash, and SO2 emissions are evident.

Figure 2. A shortwave / longwave false color composite of the same scene as Figure 1. ASTER VNIR (Band 3), SWIR (Band 4), and TIR (Band 10) are reconstructed here in the blue, green, and red color planes. The active lava flows appear red and yellow, and are visible through much of the volcanogenic plume.

In addition, ASTER VNIR Bands 3N (nadir-looking) and 3B (backward-looking) provide parallax information that permits the construction of digital elevation models (DEMs) using classic stereo photogrammetry techniques. Such products are available on-demand from the USGS Eros Data Center in Sioux Falls, South Dakota. Such data are useful in assessing post-eruption flow volumes, and in helping with pre-eruption assessments of lava flow routing paths (Figure 3).

Finally, the detection and multispectral imaging of low altitude (<10km ASL) proximal volcanic plumes with weather satellite instruments (e.g., AVHRR, GOES) at nadir resolutions ranging from 1-4km/pixel is somewhat problematic (Hufford et al., 2000). This is because the plumes themselves are often small with cross-wind dimensions of order 100m and down-wind extensions of 1-10km. Nevertheless, the detection and imaging of such plumes is important not only in the context of basic science, but also in the arena of aviation safety. Understanding the magnitude and distribution of ash and sulfur dioxide in such emissions can only be accomplished utilizing TIR-based multiband techniques at relatively high spatial resolution. In this regard, ASTER’s capabilities are unique.

ASTER, given its 14 bands between 0.5µm and 12.6µm, can directly address a number of volcanological issues that require a multispectral approach. Much of what ASTER can do in this area can also be done with Landsat ETM+ or with experimental instruments on board the EO-1 spacecraft, though at somewhat coarser spatial resolution (e.g, identification of hydrothermal alteration and mapping of spectrally contrasting weathered units with VNIR, mapping of thermal radiance from active lava flows with SWIR, monitoring summit crater activity with SWIR). ASTER, however, has the unique capability of acquiring multispectral TIR data (5 channels) at less than 100m/pixel. This scale potentially allows the deconvolution of eruption precursor hot spot data with relatively low thermal contrast (theoretical limit given by NEΔT~0.3K; practical ΔT limit~5K) with relatively modest areas (≈7x104m2), as well as geologic mapping utilizing restrahlen signature principal-component contrasts, as was pioneered with the TIMS airborne instrument (Kahle et al., 1987).

Volcanology Examples

As an example of low temperature thermal capability, warm spots associated with the currently erupting Chikurachki Volcano in the Kurile Islands were detected by ASTER as early as January 2003 before the April 2003 eruption (Pieri and Abrams, 2003). Enhanced heat flow, probably related to the subsequent explosive eruption, was detected in about a half dozen summit crater pixels, generating a consistent temperature contrast of between 5K and 10K above ambient, with temperatures hovering around the melting point of water. Between January and February 2003, the average temperature of these warm spots increased a statistically significant 1-2K. Though this analysis was carried out retrospectively, it shows the potential of ASTER as a very sensitive geo-thermometer for the detection of thermal precursors of volcanic eruptions.

SO2 is a fellow traveler with ash in explosive eruptions, and is often a key diagnostic that such an eruption has occurred. The TIR bands 10, 11, and 12 on ASTER are sensitive to an SO2 absorption band between 8 and 9 µm (Realmuto et al., 1994). Mt. Etna in Sicily is one of the world’s largest natural sources of SO2, erupting between 2500-5000MT/day during non-eruptive periods and 10,000-25,000 MT/day during paroxysmal eruptions. The NASA TOMS instrument, sensitive to UV absorption of SO2, has difficulty detecting the relatively low altitude (~3000-4000mASL) SO2 plume from Etna because of the general absorption of UV energy in the troposphere and TOMS band selection. ASTER, however, with higher spatial resolution and working on the TIR SO2 absorption feature at 8.5µm, has little difficulty in picking up the Etna SO2 plume (Figure 4).

Figure 4. One of the largest recent eruptions of Mt. Etna started on July 17, 2001 and continued until late August of that year. This ASTER image was acquired on Sunday, July 29, 2001 and shows the sulfur dioxide plume (in purple) originating form the summit, drifting over the city of Catania, and continuing over the Ionian Sea. The SO2 plume is distinguished by its absorption in ASTER TIR bands 10, 11, 12.

Even more problematic in terms of detection is the nearly constant very low altitude (<500-1000mASL) plume that has been emanating from the Pu’u O’o vent at Kilauea Volcano in Hawaii since 1983. While the emission rate is lower than Etna’s, the humid tropical air and SO2 conspire to produce coastal “vog,” or volcanic smog. Again, more weather-directed instruments like TOMS cannot see deeply enough into the troposphere to detect such a small plume. However, ASTER can clearly delineate the SO2 emission (Figure 5).

ASTER, with its pointable platform in low-earth orbit, occupies a kind of temporal niche between the high spatial resolution, multispectral nadir-looking infrequent Landsat ETM+-style observations, and the more frequent—but lower spatial resolution—weather satellite data (e.g., AVHRR and GOES). ASTER’s VNIR and SWIR scan platforms are capable of nominal off-nadir pointing up to 8.55º, and occasional off-nadir pointing up to 24º. As mentioned above, this results in a revisit interval of as short as 5 days at the equator, and much shorter revisit intervals at higher latitudes. Repeated acquisitions of ASTER data, then, could result in useful time-series observations of eruptions, if the eruptions have characteristic timescales of order 10 days or more.

A graphic example of monitoring of summit craters utilizing ASTER data is shown in Figure 6, a view of the Popocatepetl Volcano, Mexico summit crater during 2000 and 2001. Here, the summit crater exhibits thermally active pixels in both SWIR and TIR ASTER channels from September 2000 through the beginning of the following year.

An example of a longer interval time-series was the ASTER discovery that Chiliques Volcano in Chile was active. On January 6, 2002 an ASTER nighttime thermal infrared image of the Chiliques volcano showed a hot spot in the summit crater and several others along the upper flanks of the edifice, indicating new volcanic activity (Figure 7). Examination of an earlier nighttime thermal infrared image from May 24, 2000 showed no thermal anomaly.

Chiliques volcano was previously thought to be dormant. Rising to an elevation of 5778 m, Chiliques is a simple stratovolcano with a 500-m-diameter circular summit crater. Officials at the Chilean Geologic Survey reported that the summit hot spot indicated that the crater lake was heating up. During an aircraft overflight fumaroles were observed on the volcano’s flank. Such ASTER data point up the utility of time-series observations at high spatial resolution and the utility of simultaneous infrared observations.

The ASTER mission represents a fundamentally new type of observational capability with respect to studying the earth’s volcanoes. The new features of ASTER are:

1. (a) an unprecedented number of thermal infrared channels (five);
2. (b) the ability to point up to 24º off-nadir, resulting in a five day revisit interval;
3. (c) the ability to carry out simultaneous along track stereo observations;
4. (d) high spatial resolution: 15 m/pixel in the VNIR instrument, 30m/pixel in the SWIR instrument, and 90m/pixel with the TIR instrument.

References

Hufford, G., J.J. Simpson, L. Salinas, E. Barske, and D.C. Pieri, 2000, Operational considerations of volcanic ash for airlines, Bulletin of the American Meteorological. Society, 8 (4), 745-755.

Kahle A.B., 1987, Surface Emittance, Temperature, and Thermal Inertia Derived from Thermal Infrared Multispectral Scanner (TIMS) Data For Death-Valley, California, Geophysics 52 (7): 858-874.

Pieri D. and M. Abrams, 2003, ASTER pre-eruption thermal analysis of Chikurachki Volcano, in preparation for Geophysical Research Letters.

Realmuto VJ, Abrams MJ, Buongiorno MF, Pieri DC, 1994, The Use of Multispectral Thermal Infrared Image Data To Estimate the Sulfur-Dioxide Flux from Volcanoes - A Case-Study from Mount Etna, Sicily, July 29, 1986, Journal of Geophysical Research-Solid Earth 99 (B1): 481-488.

Simkin, T, and L Siebert, 1994, Volcanoes of the World (2nd Edition), Geoscience Press, Inc., Tucson, and the Smithsonian Institution, Washington, D.C., 349pp.